Dendrimers, also known as arbolols, cascade molecules, dendritic polymers, or starburst polymers, have been attracting recent attention because of their unique structure and properties. The name comes from the Greek word for tree, in reference to their branch-like structure. They are a type of 3-dimensional, hyperbranched polymer, but differ in their ability to be prepared in a highly controlled, stepwise manner with narrow molecular weight distributions, many with no variation at all in weight. Dendrimers have a central core and an outer "surface" consisting of a high number of terminal groups which can be functionalized to provide the desired properties.
There are two main strategies for synthesizing dendrimers. In the divergent approach, the molecules are built from the central core out to the periphery. The convergent approach builds the molecule from the outer periphery towards the inner core. The divergent approach is outlined below.
The dendrimer starts with a core having at least two functional end groups capable of further reaction. Monomer units, having at least three functional end groups, are added in cycles to the core via one functional end group each, building a dendrimer framework. A monomer having three such end groups is considered to have a branch multiplicity of 2, indicating that two further monomers can be added to the framework on each existing branch. Each successive cycle is called a generation. In each generation a set number of monomer units are added to the functional end groups of the dendrimer framework, leading to a series of layers. This number is an exact multiple of the multiplicity. These cycles continue until the desired number of generations are added. ##STR1##
A pioneer in this type of chemistry is Donald Tomalia, who wrote an early review summarizing the history of area (D. Tomalia, et al, Angew. Chem. Int. Ed. Engl., 29 (1990), 138-75). Recent developments were reviewed in 1997 (F. Zeng and S. Zimmerman, Chem. Rev., 97 (1990), 1681-1712).
Liu, et al (Angew. Chem. Int. Ed. Engl., 36 (1997), 2114-16, and J. Am. Chem. Soc. 119, (1997), 8720-21) describe a dendrimer covalently attached to a solid support. However, the dendrimer is not attached directly to the support but is attached to reactive sites on an intervening randomly dispersed polymer layer.
One of the many potential uses of dendrimers has been in catalysis. Metals can be attached to the surface of the dendrimers by complexation with appropriate functional groups. They combine the advantages of homogenous catalysis with heterogeneous catalysis since they have a specific number of structurally defined, catalytically active metal centers as in traditional homogenous catalysis, as well the ability for easily separation from the reaction mixture and reuse as in heterogeneous catalysis. Miedaner, et al. (Inorg. Chem., 33 (1990), 5482-5490), Knapen, et al. (Nature, 372 (1994), 659-663), and Reetz et al. (Angew. Chem. Int. Ed. Engl., 36 (1997), 1526-1528) all used small dendrimers complexed with Pd and Ni compounds as homogenous catalysts. The dendrimers were separated after reaction by precipitation and filtration. Recycling of the used catalysts was suggested but not demonstrated.
A published abstract (http://www.sfu.ca/csc98/program/00000383.htm) for the 81st Canadian Society for Chemistry Conference and Exhibition stated that a version of Tomalia's dendrimer complexed to [Rh(CO).sub.2 Cl].sub.2 was used as a catalyst for hydroformylation. The paper was withdrawn before presentation.
A need exists for a method to prepare true heterogeneous dendrimer catalysts that are efficient and can be easily removed from reaction mixtures without an added precipitation step, but can also be recycled and used multiple times.